With the wind whipping the red-clay surface at the Foro Italico, Djokovic struggled with his serve and backhand in the opening set before settling down and wearing Monaco.

"It was a very strange match. It was difficult to play with that wind. But the conditions were the same for both us, he just handled it better at the start," Djokovic said. "I was playing far too defensively and passively and he was controlling everything. He was the better player for a set and a half."

Djokovic had 22 unforced errors in the first set - including 14 backhands - to Monaco's 16, and got in only 47 percent of his first serves.

When Djokovic missed two consecutive backhands to hand Monaco the first set, he slammed his racket to the clay twice, breaking it on the second attempt, which drew a warning from the chair umpire.

Nadal will meet seventh-seeded Tomas Berdych, who eliminated No. 12 Nicolas Almagro 7-6 (3 ), 6-3. Berdych was the runner-up to Roger Federer at last week's Madrid Open on the controversial blue clay.

Also, 16th-seeded Richard Gasquet upset No. 4 Andy Murray 6-7 (1 ), 6-3, 6-2; and sixth-seeded David Ferrer beat No. 11 Gilles Simon 6-0, 7-6 (5 ). Murray skipped the Madrid Open with a back injury, and he said after the loss that it was still bothering him.

Serena Williams called for the trainer while leading 4-1 in the second set and had her left ankle re-taped. She easily won the last two games. "I wanted to loosen the tape, and so I called the trainer to retape," Serena said. "It was too tight."